"I heated the hollowed out stud with a propane torch" That's part of the problem. Propane and even the "new" MAPP gas are pretty wimpy. Back when the busted piece was still a solid stub, several oxyacetylene heat cycles would likely have likely have gotten it right out.
I'd ditch the WD-40 or PB Blaster, and get some Kroil, let it soak overnight... ...should come right out...